A location of planes that characterize a position or orientation of the pelvis, in particular the mid-sagittal plane and the frontal pelvic plane, typically are determined prior to hip surgery. The mid-sagittal plane is a vertical plane through the midline of the body that divides the body into right and left halves, while the frontal pelvic plane lies on four points of the pelvis, namely the two pubic points and two spinal points.
If, for example, a new joint cavity is to be inserted into the hip, exact positioning of the new cavity depends on, among other things, the location of the mid-sagittal plane and the location of the frontal pelvic plane. More specifically, when inserting an artificial hip joint, the orientation of the acetabular pelvic implant in relation to these two mutually orthogonal planes (i.e., the mid-sagittal plane and the frontal pelvic plane) is measured or determined.
Depending on what image data are available, these planes can be determined in different ways. For example, if navigation is based on a CT scan, these planes can be defined in the CT volume. Since the CT data set is registered in relation to the patient, these data can be transferred or converted from CT coordinates into patient coordinates. If navigation is performed without imaging methods, these planes can be directly defined on the patient using a pointer, for example.
A method and device for determining a plane of symmetry of a three-dimensional object and, in particular, for determining the mid-sagittal plane of a hip based on two x-ray or fluoroscopic recordings is known from EP 1 570 800 A1, the contents of which is hereby incorporated by reference in its entirety.
Methods for determining the position of a pelvic bone are known from US 2004/0102792 A1 and WO 2004/089192 A2, wherein positions of points on a patient are ascertained by means of a pointer.